The present invention relates, in general, to a voice coil motor control circuit for servo system control in a computer mass storage device. More particularly, the present invention relates to a voice coil motor control circuit forming part of a highly integrated, monolithic servo system controller chip of particular utility in the design and implementation of actuator or positioner control of "hard", "fixed" or "rigid" Winchester disk drives.
Disk drives are computer mass storage devices from which data may be read and/or to which such data may be written. In general, they comprise a randomly accessible rotating storage medium, or disk, on which data is encoded by various means. In magnetic disk drives, data is encoded as bits of information comprising magnetic field reversals contained on the magnetically-hard surface of the rotating disk. The bits of information are arranged serially in concentric rings called tracks, which are the curvilinear arcs described on the media surface passing by a transducer or read/write "head" whose position remains fixed for an entire revolution of the disk. Typically, disk drives will comprise multiple disks, spanned by multiple read/write heads ganged to move in unison. The collection of data tracks of all such disks corresponding to any given fixed head position is defined as a "cylinder".
The read/write head is mounted on an actuator arm which is attached to a voice coil motor capable of moving the head/arm assembly across the disk surface at very high speeds to perform seek operations. Within each track, data is usually organized into sectors of fixed length. A sector of information data may be preceded by a header and generally followed by an error correction code ("ECC"). Typically, disk controllers use the header to verify sector position before a data transfer and the ECC aids in correcting errors that may occur when data is read.
Servo control information must be available to accurately position the disk drive head over the appropriate data track. To this end, some disk drives incorporate a dedicated servo control system in which one disk, or media, surface is permanently recorded at the time of manufacture by a high precision servo writer or similar means to create a complete set of servo tracks each with a consistent pattern characteristic of a track on which it is written. The head that spans this track is then, a dedicated servo head and provides information to the position and velocity control feedback loops of the disk drive.
An alternative technique utilizes what is known as embedded servo technology which comprises embedding servo control information in the intersector gaps on the tracks of each disk surface between blocks of information data. An embedded servo system performs fine positioning of the head with respect to the track center line and continuous on track centering by reading and responding to the digital and analog information contained within the servo control sectors.
With the decreasing size or "form factor" of computer mass storage devices, particularly disk drives, there is a concomitant need to downsize the number and physical size of the associated components as well. With this trend toward ever smaller packaging, spindle motors, voice coil motor ("VCM") actuators and the like, there is an increased emphasis on reducing the power supply requirements and chip count for the associated electronics as well. At the same time, with the reduction in size of the spindle motors and actuators, ever more sophisticated control requirements are placed on the disk drive servo electronics.
Current disk drive electronic assemblies include a number of integrated circuit modules. A read/write channel and associated analog circuitry function primarily to facilitate the reading of data from and writing of data to the information data sectors of the disk. Other components provide the requisite functionality to interface the read/write channel and low level electromechanical functionality of the spindle and the actuator to a controller module. The controller module provides an interface to a host computer bus, among other things. The "low level" electromechanical functionality of the disk drive spindle motor and voice coil motor actuator for positioning the read/write heads with respect to the rotating storage media is accomplished by means of another module or subsystem which typically includes a data processor of some type, such as a digital signal processor ("DSP"), and associated microprocessor with related circuitry in the form of an application specific integrated circuit ("ASIC") and power drivers. An electronically programmable read only memory ("EPROM") containing micro instructions is typically associated with the DSP and microprocessor for use in controlling their functionality. The functionality of the read/write channel and the DSP/microprocessor module is controlled by the controller module and an associated random access memory ("RAM") which provides the host interface, buffer management, disk formatting and ECC functionality.
Current trends in the application and use of disk drives have also made it necessary to enhance the functionality of the modules and components which control smaller VCM actuators. The increased performance of computers with which the disk drives are used have placed greater demands on the disk drive to perform I/O operations more quickly and more reliably. It is also becoming increasingly desirable to provide more standardized, yet application-programmable functionality in disk drives, particularly with respect to those related to the motion control aspects of the VCM. As a consequence, it is highly desirable to provide a VCM control circuit and associated power stage which avoids the crossover distortion and "dead band" problems encountered in present designs. Moreover, it would also be highly desirable to provide a VCM control circuit which is highly integrated and capable of interfacing to a wide range of disk drive voice coil motors while simultaneously providing an effective technique for parking the actuator and heads should an undervoltage condition be detected.